1. Field of Invention
The present invention relates to a method for controlling braking of a vehicle. More particularly, the present invention relates to method for controlling braking of a vehicle, which can minimize occurrence of wheel lock and interruption of regenerative braking by determining a regenerative braking force in consideration of conditions of a road surface and optimally distributing a braking force of front wheels and rear wheels in an eco-friendly vehicle that uses an electric motor as a driving source.
2. Description of Related Art
Generally, vehicles driven using an electric motor, i.e., eco-friendly vehicles such as pure Electric Vehicles (EV), Hybrid Electric Vehicles (HEV), and Fuel Cell Electric Vehicles (FCEV) perform regenerative braking upon braking of vehicles.
A regenerative braking system of eco-friendly vehicles improves fuel efficiency by converting kinetic energy of a vehicle into electric energy during braking of a vehicle, storing the electric energy in a battery, and recycling the electric energy to drive an electric motor when a vehicle drives (recycled electric energy is reused as kinetic energy of a vehicle).
In vehicles in which the regenerative braking is performed, a generative braking cooperative control technology is needed to allow the sum of a regenerative braking torque generated in an electric motor (driving motor) and a frictional braking torque generated in a frictional braking device (hydraulic braking device) to meet a driver request braking torque.
In this case, an electric braking force by generative operation and rotation resistance of a motor, i.e., a regenerative braking force and a frictional braking force by a frictional braking device need to be appropriately distributed.
Since a vehicle equipped with a driving motor on a front wheel performs regenerative braking only at the front wheel that is a driving wheel, a regenerative braking cooperative control technology of concentrating a braking force on the front wheel is applied in order to increase the recovery rate of energy.
FIG. 1 is a view illustrating a typical method for distributing a braking force, which shows an example of distribution of a regenerative braking force and a frictional braking force according to a driver request braking deceleration D.
As shown FIG. 1, when a driver request braking deceleration D is equal to or less than a first reference value Df—1, vehicle braking is allowed to be performed only by a regenerative braking force (maximum value of a regenerative braking force is Ff—1) of a front wheel (driving wheel). On the other hand, when the driver request braking deceleration D is greater than the first reference value Df—1, the vehicle braking is allowed to be performed by the regenerative braking force Ff—1 of the front wheel and a frictional braking force (a maximum value of a frictional braking force is Fr—1 in the range of equal to or less than D1) of a rear wheel (driven wheel).
Also, when the driver request braking deceleration D is greater than a second reference value D1, the vehicle breaking is performed by allowing the sum of the regenerative braking force and the frictional braking force of the front wheel and the frictional braking force of the rear wheel to meet the total driver request braking force.
However, when a frictional coefficient of a road surface becomes lower than that of a dry road surface due to rain or snow, the breaking force concentrated on the front wheel causes a wheel lock of the front wheel to increase the braking distance. In this case, the regenerative braking is stopped while an Anti-lock Brake System (ABS) operates.
Thus, when the wheel lock of the driving wheel occurs due to a change of the frictional coefficient of the road surface during the regenerative braking operation, the motor is stopped. Accordingly, since the motor stops, the regenerative braking stops, and only the frictional braking is performed. In this case, due to the interruption of the regenerative braking, the recovery rate of energy may be reduced, and thus the fuel efficiency of a vehicle may be reduced.
In order to prevent this situation, a regenerative braking control strategy is needed to allow the regenerative braking not to stop as far as possible by distributing the braking force of the front wheel and the rear wheel in consideration of the conditions of the road surface.
In other words, although the determination of the generative braking force in consideration of the condition of the road surface and the appropriate distribution of the braking forces of the front wheels and the rear wheels have to be performed, the conditions of the road surface is not being considered at all in a related art. Accordingly, the improvement is needed.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.